1. Field of the Invention.
This invention pertains to compositions and methods for bonding gold ceramic substrates. In particular this invention relates to the field of bonding gold to ceramic substrates using a combined copper oxide and cadmium oxide composition as the major bonding agent. More in particular, this invention pertains to the field of ceramic articles of manufacture having a gold layer bonded thereto. Still further this invention relates to the field of bonding gold layers to ceramic substrates at low firing temperatures.
2. Prior Art
Compositions, and methods for bonding gold to ceramic substrates are known in the art. In addition, ceramic articles of manufacture are known which have a gold layer adhered thereto. However, in general, the mechanics of bonding gold to ceramic material has included the addition of a predetermined percentage of glass frits into the gold. The glass frits are usually incorporated in an organic binder and mixed or blended with gold powder prior to the application to the ceramic substrate. The composition (including the gold, the organic binder, and ceramic article) is heated to a temperature approaching the melting temperature of the glass. The glass frits then essentially wet the base ceramic surface and the gold and serve as a bonding agent.
In the prior art, where gold or gold alloys are mixed with organic binders containing glass frits, it has been found that an acceptable bond strength is obtained when the glass frit by weight percentage of the total composition reaches approximately 20%. However, the electrical resistivity of such compositions may be as high as 0.03 ohms/ square/mil. Since a major use of such bonds is in the production of circuit boards, the high electrical resistivity is a distinct disadvantage. When the glass frit weight percentage is diminished to approximately 2%, a lower electrical resistivity of the coating is achieved. However, the low percentage of glass frits leads to a low bonding strength, and the coating is easily removable. In practice, a trade off study is usually used where a low percentage of glass frits with a low bonding strength is traded off against a high percentage of glass frits having a high electrical resistivity. Very often, prior techniques and compositions called for a glass frit composition approaching a weight percentage of 10%. The electrical resistivity formed in the coating layer of the present invention is approximately the same as pure gold within 2% to 3%. This resistivity is substantially lower then the prior art glass frit bonded gold.
Ceramic substrate gold coatings are generally manufactured having a thickness range between 200 millionths and 1,000 millionths of an inch. This thickness range is necessary in order that a sufficient amount of glass be present in the composition in order to affect an acceptable bond. In the instant invention, where no glass or inorganic binder is used, the gold coating or layer on the ceramic substrate may be reduced to 50 millionths of an inch while maintaining an acceptable bonding action. Where glass frits are used it is evident that the cost of manufacture of coated ceramic substrates must by necessity go up while at the same time inefficiently using a natural resource.
With the addition of cadmium oxide to the copper oxide powder of the present invention the firing temperature may be significantly reduced in order to get a good bonding layer. Without cadmium oxide introduced into the mixture composition optimum bonding firing temperatures are found within the range of 1020.degree.C. to 1040.degree.C. With the addition of cadmium oxide powder the firing temperature can be reduced to 850.degree.C. with a preferred firing temperature range between 950.degree.and 1000.degree.C. The ability to lower the firing temperature permits the use of ovens which cannot reach the 1000.degree.C level as well as permitting the use of ceramic substrates which may suffer a structural degradation when their temperature is raised above the 1000.degree.C. level.
Where glass frits or other known bonds are used, the thermal conductivity of the gold layer has been found to be low. This disadvantage has the effect of producing unwanted thermal gradients between the ceramic substrate and any mounted circuitry. In the present invention where the only constituents of the bonded layer are copper oxide and cadmium oxide in combination with the gold, the overall density of the layer approximates that of the gold and further has a thermal conductivity approaching that of the pure gold.
In hybrid circuits, in some prior cases, it has been found that the glass frits contained in the gold were not compatible with the glass frits in the printed resistors. This condition possibly causes formation of bubbles and voids between the mating surfaces. From this, inaccurate readings and stresses may be built up to change the electrical characteristics of the aforementioned resistors. In the instant invention, the gold coating remains relatively inert with respect to electrical parameters.
In other prior art such as U.S. Pat. No. 3,450,545 the bonded layering includes between 4% and 35% inorganic powder which may be a glass type frit. Where any sizeable percentage of glass type frit/inorganic binder powder is used, then the density of the gold necessarily decreases.
Other prior art, such as that shown in U.S. Pat No. 2,733,167 does bond gold to a non-porous ceramic surface for various decorative purposes. However, such bonding of gold uses organic compounds of copper and do not begin with a copper oxide as in the present invention. Such prior art provides and utilizes a glazed ceramic surface or glass base to provide a coating which is only a few millionths of an inch in thickness. Such small thicknesses of gold bonding are easily removable and not applicable to the thicknesses needed in the bonding of integrated or hybrid circuitry.
Still other prior art, such as U.S. Pat. No. 3,403,043 and No. 3,429,736 provide refractory powders for ceramic bondings such as tungsten or molybdenum which must be fired in a reducing atmosphere. Where such refractory powders are fired in the air, as in the instant invention, compounds would oxidize and there would no longer be a metal layer on the ceramic substrate. In other prior art such as that shown in U.S. Pat. No. 3,647,534 there is shown the use of cuprous oxide applied to a ceramic surface for forming a weldable surface on a ceramic member. However, the ceramic is immersed in a nickel solution for depositing a layer a nickel on the reacted area and is not applicable to the gold bonded layer as in the present invention. Further, this prior art does not show the condition of cadmium oxide for incorporation into the cuprous oxide in order to lower the firing temperature of the ceramic substrate.